The use of a piezoelectric actuator as a means for converting electric power into mechanical power is known. For instance, such a piezoelectric actuator may be conveniently used to control a fuel injection valve. This is advantageous, since a piezoelectric actuator has a faster response than a conventional electromagnetic solenoid. Accordingly, control of the fuel injection valve is performed by utilizing the responsive characteristics of the piezoelectric actuator.
It is necessary to provide a drive circuit for controlling the piezoelectric actuator by supplying a voltage to the actuator. Since a piezoelectric actuator requires a relatively high drive voltage, a power source which can supply a sufficiently high voltage is required. When such a high voltage power source is not used, a drive circuit which can boost the drive voltage to the required high level is required. The latter is the conventional approach.
Conventional piezoelectric driver circuits use a LC frequency design. This type of design "matches" an inductor to a capacitor to obtain a voltage doubling effect. For example, a conventional drive circuit is explained below.
Referring to FIG. 1 V.sub.S represents the direct current voltage source, C.sub.1 a charging capacitor, C.sub.2 a discharging capacitor, L.sub.1 an inductor used at the time of charging, L.sub.2 an inductor used at the time of discharging, S.sub.1 a charging thyristor, S.sub.2 a discharging thyristor, D a diode for clamping, and PZT a piezoelectric actuator. When a trigger pulse is input to a gate G.sub.1 of the charging thyristor S.sub.1, the charging thyristor S.sub.1 is turned "on" and the piezoelectric actuator PZT is charged. In this case, since a resonant circuit is formed by the charging capacitor C.sub.1, the charging inductor L.sub.1, the charging thyristor S.sub.1, and the piezoelectric actuator PZT, approximately double the level of the supply voltage can be generated across the charging inductor L.sub.1. This voltage is supplied to the PZT, causing the PZT to expand in a longitudinal direction. When another trigger pulse is input to a gate G.sub.2 of the discharging thyristor S.sub.2, the discharging thyristor S.sub.2 is turned "on" and the PZT is discharged through the discharging inductor L.sub.2. The PZT then contracts to its original thickness.
There are, however, some disadvantages in the use of this method. Due to the large amount of energy that needs to be generated, a large inductance and capacitance are required to double the voltage value. Large inductance and capacitance values are undesirable because of the large amount of space that the inductor and capacitor require. As shown, the conventional circuit design uses an additional inductor and capacitor in the discharge path to release energy from the actuator and for energy recovery. These components add to the space occupied by the driver circuit and increase the cost of the driver circuit.
In another example, European Patent Application No. 324450 filed in Jan. 11, 1989 to Masaki Mitsuyasu et al., discloses an LC resonant circuit for driving a piezoelectric element for a fuel injection system. In Mitsuyasu's device the piezoelectric actuator is used as the charging capacitance to complete the LC circuit. Therefore, a charging capacitor is not needed. However, the capacitance value in a piezoelectric actuator varies as much as 50% from the nominal value. Since the charging current increases linearly with the piezoelectric actuator's capacitance value in a resonant circuit, control of the charging current in Mitsuyasu's design becomes difficult. Additionally, when charging resonantly, the voltage attempts to ring down after achieving a peak value. Variations in the the voltage across the actuator cause the actuator to expand and then contract in an erratic manner resulting in imperfect control of the fuel injection cycle. Also, energy is wasted by discharging the actuator to ground without recovering some of the energy released by the actuator.
A suitable piezoelectric driving apparatus is preferably a constant current device, of simple and rugged construction, and capable of functioning with multiple piezoelectric actuators with little increase in expense or complexity. The present invention is directed to overcoming one or more of the problems as set forth above.